Transfer Hydrogenation of Levulinic Acid to γ-Valerolactone and Pyrrolidones Using a Homogeneous Nickel Catalyst

Article abstract of DOI:10.1002/ejic.202001063

We report a well-defined homogeneous nickel-based catalyst using the complex [dippeNi(COD)] (dippe=1,2-bis(diisopropyl phosphino)ethane) as a catalytic precursor with high activity in the hydrogenation of levulinic acid (LVA) to yield γ-valerolactone (GVL) under relatively mild conditions (4 h, 120 °C); formic acid (FA) is the transfer hydrogenation agent in a dehydrogenation-hydrogenation process. Under optimized conditions, GVL was obtained with excellent yield (>99 %) and selectivity (>99 %). The Ni-catalyst was assessed in the LVA hydrogenation with a variety of primary amines using an excess of FA (4 eq) as hydrogen donor at 15 h and 170 °C to produce 2-pyrrolydones with excellent yield (>99 %) and fair to good selectivity (from 68 to 92 %).

Full text of DOI:10.1002/ejic.202001063

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Transfer Hydrogenation of Levulinic Acid to γ-
Valerolactone and Pyrrolidones Using a Homogeneous
Nickel Catalyst
Tamara Jurado-Vázquez,[a] Alma Arévalo,[a] and Juventino J. García*[a]
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We report a well-defined homogeneous nickel-based catalyst
using the complex [dippeNi(COD)] (dippe=1,2-bis(diisopropyl
phosphino)ethane) as a catalytic precursor with high activity in
the hydrogenation of levulinic acid (LVA) to yield γ-valerolac-
was obtained with excellent yield (>99%) and selectivity (>
99%). The Ni-catalyst was assessed in the LVA hydrogenation
with a variety of primary amines using an excess of FA (4 eq) as
°
hydrogen donor at 15 h and 170 C to produce 2-pyrrolydones
with excellent yield (>99%) and fair to good selectivity (from
68 to 92%).
°
tone (GVL) under relatively mild conditions (4 h, 120 C); formic
acid (FA) is the transfer hydrogenation agent in a dehydrogen-
ation-hydrogenation process. Under optimized conditions, GVL
Introduction
transition metals has been widely explored using noble
metals;[3b] however, catalysts based on first row-transition
metals are much less explored.
For instance, the LVA hydrogenation to GVL has been
studied via heterogeneous nickel-based catalysts,[3a,9] but only
four report transfer hydrogenation protocols.[10] In these reports,
the hydrogenation of LVA to GVL was performed under high
Renewable carbon sources such as lignocellulosic biomass
(wood, grass, and agricultural waste) are urgently needed.
Lignocellulose is one of the most abundant and inexpensive
carbon sources.[1] Levulinic acid (LVA) is an important biomass
derivative, and the United States Department of Energy
(USDOE) has included LVA on the list of the 12 most relevant
platform molecules for future research endeavours since 2004.[2]
Levulinic acid can be transformed to a range of higher-value
molecules such as γ-valerolactone (GVL) and pyrrolidones like
alkyl-5-methyl-2-pyrrolidones.[3] The conversion from LVA to
GVL is particularly relevant in several venues like bio-based
solvents, fuels, and as a food additive.[4] Pyrrolidones are
produced from LVA by a reductive amination and are used as
industrial solvents, surfactants, and intermediates in the syn-
thesis of functional compounds such as printing ink and fibre
dyes.[5] Both processes involve hydrogenation, and in this
context, formic acid (FA) is one of the most promising hydrogen
sources.
FA is also a biomass product obtained from lignocellulose;
both FA and LVA are produced in equal proportions from
hemicellulose acid catalysis.[1b,6] FA is an ideal hydrogen donor
in LVA reduction. In the recent years, FA has become an
important and safe H2 source that has been widely explored[7]
mainly via the use of noble metals like Au, Pd or Ir as catalysts.[8]
Due to the relevance of biomass derivatives, the develop-
ment of inexpensive and efficient catalytic protocols are needed
in this field, and the development of catalysts based on
°
temperatures (above 170 C) and a large excess of hydrogen
transfer agent. Closely-related homogeneous nickel-based cata-
lysts were used for the hydrogenation of ketones.[11,12] While this
manuscript was written, a report using [Ni(OAc)2] as catalytic
precursor for the transformation of LVA to GVL was disclosed,
along with chelating phosphines and hydrogen at high pressure
(15 atm); however, the homogeneous nature of such catalytic
mixture was not established.[13] In 2016, our group reported the
use of a nickel complex [(dcype)Ni(COD)] as a catalytic precursor
for the transfer hydrogenation of ketones employing ethanol as
a hydrogen donor with excellent yields.[12]
The conversion of LVA to 2-pyrrolidones has been even less
explored using Ni as a catalayst: There are only a few related
reports.[14] In 2017, Qin et al. reported the use of nickel catalysts
supported on carbon nanotubes with excellent yield and
selectivity using a high pressure of hydrogen (30 Bar).[14a] In
2018, Lawrence and colleagues reported the synthesis of 2-
pyrrolidone from LVA using FA as a reducing agent and Ni-
Raney as a catalyst with high temperatures and very good
conversion.[14b] More recently, Jagadees and colleagues reported
the use of heterogeneous nickel-based catalysis in ketone
amination and hydrogenation in 2020 with excellent yield.[15]
Here, we describe the first study of LVA hydrogenation
using a well-defined Ni homogeneous catalyst with excellent
yields and selectivity using FA as a transfer hydrogen agent.
GVL was produced using relatively mild reaction conditions
with excellent yields and selectivity. We further studied LVA
hydrogenation with primary amines and FA to obtain 2-
pyrrolydones.
[a] T. Jurado-Vázquez, Dr. A. Arévalo, Prof. Dr. J. J. García
Facultad de Química, Universidad Nacional Autónoma de México,
Mexico City, 04510, Mexico
E-mail: juvent@unam.mx
https://quimica.unam.mx/perfil/juventino-garcia-alejandre/
Supporting information for this article is available on the WWW under
https://doi.org/10.1002/ejic.202001063
Part of the “Inorganic Chemistry in Latin America” Special Collection.
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followed by their hydrogenation to yield the corresponding 2-
pyrrolidone.
Thus, [dippeNi(COD)] was tested as a catalytic precursor in
the levulinic acid conversion to 2-pyrrolidone using benzyl-
amine as model substrate. Based on LVA hydrogenation
conditions, the 2-pyrrolidone synthesis conditions were opti-
mized in THF as solvent (Table 2). The best conversion to 2-
Results and Discussion
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Catalytic transfer hydrogenation of LVA with nickel
compounds
The catalytic hydrogenation of LVA was evaluated using
[Ni(COD)2] as a catalytic precursor and dippe as ancillary ligand
to form [dippeNi(COD)] in situ. The initial experimental setup
included the use of a variety of solvents (Table S1). When water
was tested as a solvent, the conversion of LVA to GVL decreased
from >99% to 20% probably due to the low solubility of both
[Ni(COD)2] and [dippeNi(COD)]. The addition of a small amount
of THF facilitated the solubility of the reaction mixture. The
importance of the use of dippe as an ancillary ligand was tested
by doing the reaction in the absence of dippe (Table S2,
Entry 2). In this experiment, no conversion from LVA to GVL was
observed. The reaction conditions (time and temperature) and
the hydrogen transfer agent (formic acid) were optimized (see
Table 1). Thus, the optimized reaction conditions, GVL could be
obtained with an excellent yield, performing the reaction at
°
pyrrolidinone-1-benzyl-5-methyl (3p) was obtained at 170 C at
15 hours with an excellent yield (Table 2, entry 10).
A competitive reaction between GVL and product 3p was
observed during the catalytic process optimization (Scheme S1).
To reduce this competition, the water generated in the catalytic
process was trapped using molecular sieves (Table 2, entries 9
and 10); the conversion to product 3p increased from 74 to
83%. A mercury drop test was performed (Table 2, entry 13)
confirming that the reaction to obtain 2-pyrrolidones occurs via
homogeneous catalysis.
Of note, in the production of 2-pyrrolidone, the order of
addition for reagents is important because the formic acid
reacts with benzylamine, and the conversion of the product 3p
decreases with formamide-N-(phenylmethyl) produced. To pre-
vent the formation of this formamide, LVA and benzylamine
were mixed first with the molecular sieves and stirring to obtain
2p. After 5 minutes, the mixture of [Ni(COD)2] and dippe was
added followed by formic acid.
Due to the industrial relevance of 2-pyrrolidones, a scope
using different primary amines was carried out under the
above-described optimized reaction conditions (Tables S3–S7).
Some selected data to yield product 3p are presented in
Table 3. The 2-pyrrolidone yields are good (68 to 92%);
selectivity was also good. The best conversion was obtained
using cyclohexylamine, and the lowest conversion was with
butylamine.
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°
120 C for 4 h, using a mixture of H2O:THF (5:1) as a solvent,
4 mol% of [Ni(COD)2], and 8 mol% of dippe. It is not necessary
to use an excess of hydrogen transfer agent FA. This reaction
occurred homogeneously as confirmed by a Hg(0) drop test
(Table 1 entry 11) without any loss of activity.
To study further the catalytic process, [dippeNi(COD)] was
prepared as previously reported[12,16] and was used with a
catalytic precursor (Table 1, entry 12) giving the same catalytic
activity.
Catalytic synthesis of 2-Pyrrolidone
As previously mentioned, relevant LVA derivatives are the 2-
pyrrolidones formed by a levulinic acid and primary amine
condensation to produce a 2-pyrroline (2p) or an imine
Mechanistic insights
It has been reported that nickel complexes containing the
fragment “(dippe)Ni” in the presence of ketones form stable
[(dippe)Ni(η2-O,C-ketone)] compounds.[12,17] Thus, a stoichiomet-
ric reaction between [dippeNi(COD)] and LVA was made and
monitored by 31P{1H} and 1H NMR spectroscopy (Scheme S2 and
Scheme S3) with the aim of identifying intermediates. Two
Table 1. Optimization of reaction conditions.
Entry
FA
[eq]
Conditions
Time [h]
Yield [%][d]
GVL
doublets in the 31P{1H} NMR at δ 65.03 and 64.46 ppm (2JP-P
=
°
Temp. [ C]
Total
75 Hz) are characteristic for Ni(0) compounds and were assigned
to [(dippe)Ni(η2-O,C-LVA)] as labelled as [2] in Scheme 1, making
this step a strong difference of the mechanistic proposal and
DFT calculations recently reported,[13] where the addition of the
carboxylic acid was postulated.
Furthermore, the interaction between formic acid and
[dippeNi(COD)] was also studied, and this reaction was moni-
tored by GC-MS (Scheme S4) and 1H NMR (Scheme S5). As
expected, formic acid dehydrogenates in the presence of the
nickel catalyst to generate H2 and carbon dioxide. Thus, LVA
hydrogenation to GVL is the same with a H2 atmosphere or FA
(Table 1 entry 10). Therefore, a dehydrogenation-hydrogenation
mechanism is proposed (Scheme 1).
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2
1
–
1
1
10
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10
8
6
4
2
4
4
4
140
120
100
120
120
120
120
120
120
120
120
120
>99
>99
26
>99
>99
>99
50
>99
>99
>99
>99
>99
>99
>99
26
>99
>99
>99
50
>99
>99
>99
>99
>99
9
10[a]
11[b]
12[c]
4
4
[a] Without HCOOH, using H2 as a reductant agent. [b] Homogeneous test
(Hg(0) drop). [c] Using [dippeNi(COD)] as catalytic precursor. [d] All yields
were determined by GC-MS, using GVL as an internal standard.
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Table 2. Synthesis of 2-pyrrolidones and optimization of the reaction conditions.
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Entry
Conditions
Yield [%][e]
GVL
°
T [ C]/t [h]
2p
3p
Total
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8
140/8
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31
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99
99
99
99
99
99
140/10
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140/15
130/13
130/15
150/15
160/15
170/15
170/15
170/17
170/15
170/15
170/15
n.d.
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38
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4
10
1
53
7
46
9
10[a]
11[a]
12[a,b]
13[a,c]
14[a,d]
2
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[a] Using molecular sieve 4 Å. [b] 2eq of FA. [c] Drop Hg(0) test. [d] Test with [Ni(COD)2] 2 mol% and dippe 4 mol%. [e] All yields were determined by GC-MS,
using GVL as internal standard.
Table 3. Scope of 2-pyrrolidones using different primary amines.
Amine
Conditions
Time
[h]
Yield [%][a]
GVL
Temp.
2p
10
3p
83
Other
n.d.
Total
°
[ C]
15
170
7
>99
15
15
16
150
170
170
19
14
6
12
6
68
80
92
n.d.
n.d.
1
99
>99
>99
1
16
170
2
26
72
n.d.
>99
12
3
15
170
6
2
75
98
[a] All yields were determined by GC-MS, using GVL as internal standard.
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Scheme 1. Mechanistic proposal for LVA hydrogenation catalyzed by nickel.
In the reaction sequence in Scheme 1, the catalytic active Experimental Section
species [dippeNi(COD)] [1] is obtained first by mixing dippe and
[Ni(COD)2] with a small amount of THF before adding them to
the reaction mixture. We then propose the formation of [2] by a
side-on LVA coordination via the ketone moiety. The next step
is the oxidative addition of H2, which is generated from FA to
yield [3]. A hydride addition to obtain [4] is proposed and finally
there is a reductive elimination to release 4-hydroxypentanoic
acid. This is further condensed to obtain γ-valerolactone and
water.
General considerations
Unless otherwise stated, all processes were performed using
standard Schlenk techniques in an inter-gas/vacuum double
manifold or under argon atmosphere (Praxair 99.998); this used a
MBraun Unilab Pro SP glovebox (<1 ppm H2O and O2). All liquid
reagents were purchased as reagent grade and degassed before
use. Regular THF (J.T. Baker) was dried and degassed in a MB-SPS-
800. Water was distilled, deionized, and degassed under an argon
flow before use. Levulinic acid (98% purity), formic acid (reagent
grade �95%), benzylamine (99% purity), n-butylamine (98%
purity), sec-butylamine (98% purity), cyclohexylamine (98% purity),
4-fluorobenzylamine (97% purity), and 2-propen-1-amine (98%
purity) were purchased from Sigma-Aldrich; deoxygenated [Ni-
(COD)2] (98%) was from Strem Chemicals Inc. Ultrahigh-purity
hydrogen (5.0, Praxair) was used. Deuterated solvents for NMR
experiments were purchased from Cambridge Isotope Laboratories
and were stored under 4 Å molecule sieves for 24 h before use.
NMR spectra were recorded at room temperature on the following
spectrometers: Varian V NMRS 400 MHz and JEOL 600 MHz. All
reagents for the catalytic reactions were loaded in the indicated
glove box. The GC-MS determinations were made using an
Agilent5975C system equipped with a 30-m DB-5MS capillary
column (0.25 mm i.d.; 0.25 mm).
Conclusions
This report shows that levulinic acid transfer hydrogenation was
carried out satisfactorily using a well-defined catalytic precursor
[dippeNi(COD)]; formic acid was a transfer hydrogen agent. This
process is selective to γ-valerolactone and offers a very good
yield (>99%) using relatively mild reaction conditions (4 h,
120 C). The reaction monitoring was with 31P{1H} NMR and
°
detected an important catalytic intermediate [(dippe)Ni(η2-O, C-
LVA)]. Additionally, the same catalytic precursor allowed the
production of 2-pyrrolidones from LVA and primary amines in
good yields and very good selectivity.
Optimization of reaction conditions to obtain γ-valerolactone
The reactions were performed using
equipped with a Rotaflo valve and a magnetic stirring bar. This step
a 50 mL-Schlenk flask
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used [Ni(COD)2] (3.8 mg, 0.0138 mmol), dippe (7.2 mg,
Reaction of [dippeNi(COD)] and LVA monitored by NMR
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0.0276 mmol), LVA (40 mg, 0.344 mmol), FA (66.8 mg, 1.380 mmol),
THF (3 mL), or a mix H2O/THF (2.5/0.5 mL). The [Ni(COD)2] and dippe
were mixed using THF first independently; LVA and FA were
solubilized with H2O or THF and added to the Schlenk flask with
stirring. A mixture of [Ni(COD)2] and dippe were added. The
reaction mixture was heated at different temperatures in a silicon
oil bath for different periods of time. This reaction was assessed
with different amounts of formic acid (66.8–16.7 mg, 1.380–
0.344 mmol), [Ni(COD)2], and ligand dippe. All reactions were
analysed by GC/MS.
This step used [dippeNi(COD)] (10 mg, 0.023 mmol) and levulinic
acid (2.7 mg, 0.023 mmol) in THF-d8 (1 mL). All reactants were
added into a NMR tube (WILMAD with J. Young valve), and
1
analysed by 31P{1H} and H NMR at r.t. The first analysis was right
after mixing (t=0) with a subsequent analysis 1 h later at room
°
temperature. The sample was finally heated at 100 C for 1 h. This
analysis was performed using a JEOL 600 MHz NMR spectrometer.
Formic acid conversion to CO2 and H2
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The conversion of FA to CO2 and H2 was monitored with both GC-
MS and 1H-NMR. The 1H-NMR study was performed using [Ni(COD)2]
(1 mg, 0.0036 mmol), dippe (1.9 mg, 0.0073 mmol), and FA
(33.1 mg, 0.73 mmol) in THF-d8 (1 mL); all reactants were added into
a NMR tube (WILMAD with J. Young valve) and monitored by H
NMR at room temperature. The first analysis was carried out at t=
Use of H2 to obtain γ-valerolactone
The reaction was performed using a 50-mL Schlenk flask equipped
with a Rotaflo valve and a magnetic stirring bar. It was loaded with
[Ni(COD)2] (3.8 mg, 0.0138 mmol), dippe (7.2 mg, 0.0276 mmol), LVA
(40 mg, 0.344 mmol). All reagents were loaded in a globe box
except H2, which was added using a gas/vacuum double manifold.
1
°
0. The NMR tube was then heated at 120 C, and the reaction
mixture was analysed by 1H NMR at r. t. for 4 hours.
°
The mixture was heated at 120 C in a silicon oil bath for 8 h.
For GC-MS, this study was performed using a 50-mL Schlenk flask
equipped with a Rotaflo valve and a magnetic stirring bar. The flask
was loaded with [Ni(COD)2] (8 mg, 0.0288 mmol), dippe (15.2 mg,
0.0584 mmol), and FA (264.8 mg, 5.84 mmol) in THF (2 mL). The first
analysis was right after mixing (t=0). The sample was then heated
Hg drop test
This reaction was performed as described above but with the
addition of 61.0 mg (0.306 mmol) of Hg (0), followed by heating at
°
at 120 C and analysed by GC-MS every 4 hours.
°
120 C for 8 h. The reaction mixture was filtered with Celite and
analysed by GC/MS.
Acknowledgments
Optimization of reaction conditions to obtain 2-pyrrolidinone
1-benzyl-5-methyl
We thank CONACyT (A1-S-7657) and DGAPA-UNAM (IN-200119)
for financial support. T. J.-V. also thanks CONACyT for a Ph.D.
grant (696382).
The reactions were performed using
a 50 mL Schlenk flask
equipped with a Rotaflo valve and a magnetic stirring bar. This
work used [Ni(COD)2] (3.8 mg, 0.0138 mmol), dippe (7.2 mg,
0.0276 mmol), LVA (40 mg, 0.344 mmol), FA (66.8 mg, 1.380 mmol),
benzylamine (36.8 mg, 0.344 mmol), and THF (3 mL). First, 4 Å
activated molecular sieves (500 mg) were added to the Schlenk
flask, and then LVA and benzylamine were solubilized with THF,
mixed, and added to the Schlenk flask and stirred for 5 min.
Meanwhile, [Ni(COD)2] and dippe were mixed independently using
THF and added to the Schlenk flask; after 5 minutes, FA was added
into the reaction. The reaction mixture was heated to different
temperatures in an oil bath for different periods of time. All
reactions were filtered over Celite and analysed by GC/MS.
Conflict of Interest
The authors declare no conflict of interest.
Keywords: Homogeneous catalysis · Levulinic acid · Nickel ·
Transfer hydrogenation
2-Pyrrolidone scope
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